PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
830AM EST THURSDAY NOV 21 2013
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2013
THE DECEMBER 2013 OUTLOOK REFLECTS SHORT-TERM FORECASTS OF EXPECTED WEATHER
CONDITIONS DURING THE EARLIEST FEW DAYS OF THE MONTH AND THE INFLUENCE OF OTHER
MODES OF VARIABILITY IN THE CLIMATE SYSTEM.
RECENT OCEANIC AND ATMOSPHERIC OBSERVATIONS ACROSS THE EQUATORIAL CENTRAL
PACIFIC OCEAN CONTINUE TO SUGGEST ENSO NEUTRAL CONDITIONS. SEA SURFACE
TEMPERATURE (SST) ANOMALIES WERE NEAR ZERO ACROSS MUCH OF THE EQUATORIAL
PACIFIC, WITH SLIGHTLY ABOVE-NORMAL TEMPERATURES JUST WEST OF THE DATE LINE AND
SLIGHTLY BELOW-NORMAL TEMPERATURES NEAR THE COAST OF SOUTH AMERICA. THE
LOW-LEVEL AND UPPER-LEVEL WINDS ARE BOTH NEAR AVERAGE ACROSS MOST OF THE
EQUATORIAL PACIFIC. LARGE SCALE CONVECTIVE ACTIVITY OVER PARTS OF INDONESIA
AND THE FAR WESTERN PACIFIC CONTINUE TO BE ENHANCED, WHILE CONVECTION OVER THE
CENTRAL PACIFIC REMAINS SUPPRESSED. TAKEN TOGETHER, THESE ATMOSPHERIC AND
OCEANIC CONDITIONS ARE CONSISTENT WITH ENSO NEUTRAL CONDITIONS. STATISTICAL
AND DYNAMICAL MODELS PREDICT SST'S IN THE ENSO SENSITIVE REGIONS OF THE
EAST-CENTRAL PACIFIC TO BE NEAR NORMAL FOR THE NEXT FEW MONTHS, SUGGESTING THAT
THE ENSO STATE WILL REMAIN NEUTRAL IN DECEMBER, 2013.
THE TRADITIONAL STATISTICAL TOOLS, INCLUDING OCN, YIELD VERY LITTLE SIGNAL FOR
MOST OF THE COUNTRY FOR THE MONTH OF DECEMBER, ALTHOUGH IT IS SLIGHTLY STRONGER
THAN FOR NOVEMBER. SOME INFORMATION WAS GLEANED FROM THE NMME AND IMME FOR THE
DECEMBER FORECAST. THE STATE OF SOIL MOISTURE HAS LITTLE INFLUENCE DURING THIS
TIME OF THE YEAR, WHILE ANOMALOUS SNOW COVER CAN PLAY A ROLE. ACCORDING TO THE
RUTGERS GLOBAL SNOW LAB, THERE ARE POCKETS OF ABOVE-AVERAGE SNOW COVER ACROSS
THE NORTHERN GREAT PLAINS.
THE DECEMBER TEMPERATURE OUTLOOK INDICATES ELEVATED ODDS FOR ABOVE-NORMAL
MONTHLY MEAN TEMPERATURES ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN ROCKIES,
SOUTHWEST, AND SOUTHERN GREAT PLAINS. THAT FORECAST IS SUPPORTED BY DYNAMICAL
MODEL OUTPUTS. TRENDS FAVOR ABOVE-NORMAL MONTHLY MEAN TEMPERATURES ACROSS THE
NORTH SLOPE OF ALASKA WHILE REDUCED SEA ICE COVERAGE AND ABOVE-NORMAL SSTS NEAR
MOST OF THE WEST COAST OF ALASKA SUPPORT ELEVATED ODDS FOR ABOVE-NORMAL
TEMPERATURES ACROSS MUCH OF WESTERN AND NORTHERN ALASKA. ELEVATED ODDS FOR
BELOW-NORMAL MONTHLY MEAN TEMPERATURES ARE INDICATED ACROSS THE NORTHERN GREAT
PLAINS AND NORTHERN ROCKIES. THIS PATTERN IS CONSISTENT WITH NEUTRAL ENSO
CONDITIONS AND SLIGHTLY ELEVATED SNOW COVER.
THE PRECIPITATION OUTLOOK FOR DECEMBER INDICATES ELEVATED CHANCES FOR
BELOW-MEDIAN PRECIPITATION ACROSS THE SOUTHERN GREAT PLAINS AND PORTIONS OF THE
SOUTHEAST. ELEVATED ODDS FOR ABOVE-MEDIAN PRECIPITATION ARE INDICATED ACROSS
THE NORTHERN GREAT PLAINS AND NORTHERN ROCKIES, AS WELL AS ACROSS WESTERN AND
CENTRAL ALASKA, WITH BOTH OF THOSE REGIONS CONSISTENT WITH DYNAMICAL MODEL
OUTPUT.
THE MAIN FACTORS WHICH USUALLY INFLUENCE THE MONTHLY CLIMATE OUTLOOK INCLUDE:
1) EL NINO AND LA NINA - WHICH COMPRISE ENSO. IMPACTS OF THESE EVENTS ARE
SUMMARIZED BY SEPARATING 3-MONTH OBSERVATIONS FROM THREE OR MORE DECADES INTO
EL NINO, NEUTRAL AND LA NINA SETS, AVERAGING EACH SEPARATELY, AND THEN
COMPUTING ANOMALIES. THESE ARE CALLED "COMPOSITES" AND USED AT TIMES TO
SUBJECTIVELY MODIFY THE FORECAST.
2) TRENDS - APPROXIMATED BY THE OCN TOOL AS THE DIFFERENCE BETWEEN THE MOST
RECENT 10-YEAR MEAN OF TEMPERATURE OR 15-YEAR MEAN OF PRECIPITATION FOR A GIVEN
LOCATION AND TIME OF YEAR AND THE 30-YEAR CLIMATOLOGY PERIOD (CURRENTLY
1981-2010).
3) THE TROPICAL 30-60 DAY OSCILLATION - CALLED THE MADDEN JULIAN OSCILLATION
(MJO) - AFFECTS CLIMATE VARIABILITY WITHIN SEASONS.
4) THE ARCTIC OSCILLATION (AO), THE SIMILAR NORTH ATLANTIC OSCILLATION (NAO),
AND THE PACIFIC NORTH AMERICAN (PNA) PATTERNS - WHICH AFFECT THE TEMPERATURE
ANOMALY PATTERN ESPECIALLY DURING BOREAL WINTER AND ARE GENERALLY CONSIDERED TO
HAVE LOW PREDICTABILITY.
5) THE PACIFIC DECADAL OSCILLATION (PDO) - AN ENSO-LIKE PATTERN OF CLIMATE
VARIABILITY AFFECTING THE TROPICS AND THE NORTH PACIFIC AND NORTH AMERICAN
REGIONS, BUT WHICH VARIES ON A MUCH LONGER TIME-SCALE THAN ENSO.
6) PERSISTENTLY DRY OR WET SOILS IN THE SPRING AND SUMMER AND SNOW AND ICE
COVER ANOMALIES IN THE WINTER. THESE FACTORS TEND TO PERSIST FOR LONG PERIODS
AND ACT AS A KIND OF MEMORY IN THE CLIMATE SYSTEM.
7) STATISTICAL FORECAST MODELS - CANONICAL CORRELATION ANALYSIS (CCA),
SCREENING MULTIPLE LINEAR REGRESSION (SMLR), CONSTRUCTED ANALOG (CA), AND
ENSEMBLE CCA (ECCA).
8) DYNAMICAL FORECAST MODELS - INCLUDING THE NCEP CLIMATE FORECAST SYSTEM
VERSION 2 (CFS) AND FOR THE ZERO-LEAD UPDATE FORECASTS THE NCEP GLOBAL ENSEMBLE
FORECAST SYSTEM (GEFS). AN EXPERIMENTAL FORECAST SYSTEM, THE NATIONAL
MULTI-MODEL ENSEMBLE (NMME), COMPRISED OF SEVERAL DYNAMICAL MODELS IS ALSO USED.
FORECASTER: MATTHEW ROSENCRANS
THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1981 AND 2010, FOLLOWING
THE WORLD METEOROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3
COMPLETE DECADES AS THE CLIMATE REFERENCE PERIOD. THE PROBABILITY ANOMALIES
FOR TEMPERATURE AND PRECIPITATION BASED ON THESE NEW NORMALS BETTER REPRESENT
SHORTER TERM CLIMATIC ANOMALIES THAN THE FORECASTS BASED ON OLDER NORMALS.
AN UPDATED MONTHLY OUTLOOK... FOR DEC WILL BE ISSUED ON SAT NOVEMBER 30 2013
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD.
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